Chromatin organization and its relation to replication and histone acetylation during the cell cycle in barley

Abstract.

We have studied the replication time, nuclear organization and histone acetylation patterns of distinct chromatin domains [nucleolus organizers (NORs) , centromeres, euchromatin and heterochromatin] of barley during the cell cycle. The Rabl orientation of chromosomes, with centromeres and telomeres located at opposite nuclear poles, was found to be maintained throughout interphase. Replication started at the rDNA loci within nucleoli and then proceeded from the euchromatic distal chromosome regions toward the heterochromatic pole. Centromere association frequently occurred in mid- and late S-phase, i.e., during and after centromere replication. Euchromatin, centromeres and heterochromatin were found to be enriched in acetylated histone H4 (except for lysine 16) during their replication; then deacetylation occurred. The level of deacetylation of H4 in heterochromatin was more pronounced than in euchromatin. Deacetylation is finished in early G2-phase (lysine 8) or may last until mitosis or even the next G1-phase (lysines 5 and 12). The NORs were found to be most strongly acetylated at lysines 5 and 12 of H4 during mitosis, independently of their potential activity in nucleolus formation and rDNA transcription. The acetylation pattern of chromosomal histone H3 was characterized by low acetylation intensity at centromeres (lysines 9/18) and pericentromeric regions (lysine 14) and more intense uniform acetylation of the remaining chromatin; it remained fairly constant throughout the cell cycle. These results have been compared with the corresponding data published for mammals and for the dicot Vicia faba. This revealed conserved features as well as plant- or species-specific peculiarities. In particular, the connection of acetylation intensity of H4 at microscopically identifiable chromatin domains with replicational but not with transcriptional activity during the cell cycle seems to be conserved among eukaryotes.